© Copyright 1996 American Chemical Society
JULY 10, 1996 VOLUME 12, NUMBER 14
Letters Using Epitaxy To Form Insoluble, Ultrathin Films of Block Copolymers Masahito Sano,* Momoyo Wada, Asako Miyamoto, and Susumu Yoshimura Pi-Electron Materials ProjectsERATO, JRDC, 43 Miyukigaoka, Tsukuba, Ibaraki 305, Japan Received October 31, 1995. In Final Form: May 17, 1996X A new method of assembling functional films by epitaxy of diblock copolymers is reported. The fundamental architecture consists of a crystalline block that is used to tether the whole chain onto a substrate surface by epitaxial adsorption, and a noncrystalline block that provides functional groups. Epitaxial adsorption is realized by polymerization-induced epitaxy, where solution polymerization of monomers directly induces epitaxial growth of chains on the substrate surface. After cationic polymerization of tetrahydrofuran (THF) with the presence of graphite in a reaction mixture, a mixture of THF and 3,3-bis(chloromethyl)oxacyclobutane (BCMO) were introduced to the reaction mixture to form PTHFblock-P(THF-BCMO). Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the presence of the block copolymer on graphite even after rigorous rinsing of the substrate surface. Atomic force microscopy scanned over micrometer ranges exhibited the same island features as that of PTHF homopolymer film. But these crystalline islands are covered by ultrathin, highly viscous layers that are insoluble. Molecular resolution images by scanning tunneling microscopy showed that the features on the copolymer film are the same as that of an epitaxially adsorbed monolayer of PTHF homopolymer. No film remained on the graphite surface if the graphite was simply immersed in the copolymer solution and was rinsed similarly. These results indicate that the insoluble film of diblock copolymer was formed during polymerization, in which the crystalline PTHF block binds to the surface by epitaxial adsorption and the noncrystalline P(THF-BCMO) block is free of surface attachment.
Introduction Formation of an ultrathin film of organized molecules on a solid substrate is one of the most fundamental steps commonly employed to construct supramolecular assemblies. For instance, Langmuir-Blodgett films are made by transferring compressed amphiphilic molecules spread at the air-water interface onto a hydrophilic or hydrophobic substrate.1 So-called hydrophobic interactions as well as molecular packing and stiffness are considered to be important factors for formation of stable films. In self-assembled monolayers, molecules with a reactive end are often irreversibly adsorbed on a suitable substrate and are densely packed to gain orientational X
Abstract published in Advance ACS Abstracts, June 15, 1996.
(1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991.
S0743-7463(95)00543-9 CCC: $12.00
order.2 The molecules are anchored onto the surface by either direct chemical bonding or electrostatic interactions through networking.3 In the alternating adsorption method, a polyelectrolyte in a charged state is allowed to adsorb onto an ionic surface by electrostatic interactions.4 Here, we introduce another principle of film formation leading to supramolecular architecture. The new method is based on epitaxy of diblock copolymers on atomically flat surfaces. Recently, it has been shown that in situ polymerization of monomers near a flat surface induces formation of crystalline polymer films, often adsorbed epitaxially.5,6 In order to form these layers, the growing (2) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (3) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 12161218. (4) Lvov, Y. M.; Decher, G. Crystallogr. Rep. 1994, 39, 628-647. (5) Sano, M.; Sandberg, M. O.; Yamada, N.; Yoshimura, S. Macromolecules 1995, 28, 1925-1937.
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polymer must possess a high degree of two-dimensional crystallinity. This phenomenon offers a possibility to control epitaxial adsorption of multiblocked chains through polymerization reactions. By planning an order of polymerization according to crystallinity of each block, it may be possible to realize selective epitaxial growth. Fixation of amorphous chains by van der Waals interactions adds a new scheme to form tethered chains on a solid surface, in addition to the commonly employed techniques of graft polymerization or spontaneous adsorption onto funtionalized surfaces.7 Previous studies on homopolymers demonstrate that, unlike ordinarily recrystallized or spontaneously adsorbed polymers, the epitaxially adsorbed films exhibit excellent resistance to dissociation in common organic solvents. Insolubility may be a result of the close packed states, both within the film plane and between the chains and the substrate, of epitaxially adsorbed molecules that prevent solvent molecules from solvating a significant portion of the chain simultaneously. This insolubility allows us to isolate the epitaxially adsorbed films from spontaneously adsorbed layers by simple rinsing. This property is also a focus of the present study, since it allows us to chemically modify the functional group of the block copolymer after forming the film. We have chosen poly(tetrahydrofuran) (PTHF) as a crystalline block, since the epitaxial structures of PTHF by the present method has been already studied in detail.6 The same polymerization as employed previously, namely, ring-opening polymerization of THF with a cationic catalyst, was used in this study to control epitaxial adsorption. This scheme led to an idea of introducing another kind of cyclic ether as a comonomer to form a random copolymer, which is used as a noncrystalline block. Experimental Section To an 80% solution of tetrahydrofuran (THF) in dichloromethane (60 mol % of total monomers) containing a freshly prepared substrate and a magnetic stir bar was added no more than 1 mol % of BF3O(C2H5)2 and 0.5 mol % of epichlorohydrin in N2 atmosphere at 0 °C.8 After the reaction was allowed to proceed for 24 h, 20 mol % each of THF and 3,3-bis(chloromethyl)oxacyclobutane (BCMO) were added to the reaction mixture. One week was allowed before quenching the polymerization with methanol. The substrate was taken out of the reaction mixture and was rinsed with chloroform, cyclohexanon, dimethylformamide, 1-methyl-2-pyrrolidone, THF, ethanol, and water, refreshing the solvents frequently over a period of several days. Rinsing was repeated longer than the period necessary for the spontaneously adsorbed materials to be washed away completely (which constitutes a control sample without polymerization). We have used highly oriented pyrolytic graphite and hydrogen-terminated Si(111) (H:Si(111), cleaned by the RCA method, followed by a final treatment by NH4F) as the substrates. Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to identify the molecular films. Atomic force microscopy (AFM, Topo Metrix) in the constant force mode was operated in air using a Si3N4 cantilever (Park Instrument) with an estimated force on the order of 10-8 N. Noncontact AFM was performed in air using Si cantilever that was supplied by the manufacturer with a typical resonance frequency around 140 kHz. Scanning tunneling microscopy (STM) was performed with a home-made microscope6 in the fast scanning mode (producing several images per second) in ambient with Pt/Ir tip. A typical set current was 200 pA with a bias of 1 V. No image processing has been performed other than filtering high-frequency noise (corresponding to features (6) Sano, M.; Sasaki, D. Y.; Kunitake, T. Macromolecules 1992, 25, 6961-6969. (7) Halperin, A.; Tirrel, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31-71. (8) Saegusa, T.; Imai, H.; Furukawa, J. Makromol. Chem. 1962, 53, 203-206.
Letters
Figure 1. FTIR spectra from (a) the graphite surface that has been subjected to a synthesis of PTHF-block-P(THF-BCMO) block copolymer and has been washed repeatedly by solvents and (b) a cast film of the block copolymer on gold. A peak around 2350 cm-1 due to CO2 is omitted for clarity. smaller than approximately 0.1 Å) on the images. All measurements were conducted at room temperature.
Results and Discussion The block polymer isolated from the solution phase showed IR and NMR spectra consistent with the previously reported PTHF and a random copolymer of THF and BCMO, P(THF-BCMO).9 Ring-opening polymerization of THF by BF3O(C2H5)2 is thought to proceed with oxonium ions and to involve a propagation-depropagation equilibrium, resulting in a “living-like” reaction and a narrow molecular weight distribution.10 Copolymerization of THF and BCMO has been studied in detail and the product of monomer reactivity ratios is nearly 1.9 Gel permeation chromatography (polystyrene standard) on the polymer isolated from the solution indicated a unimodal distribution with an average molecular weight of 18 000 and a polydispersity of 1.4. Although these values are not critical in the present study, they become important for a study of the film morphology. Differential scanning calorimetry gave a glass transition temperature of -60 °C (that of PTHF homopolymer is -84 °C and of PBCMO homopolymer is -8 °C),11 but no definite melting (9) Saegusa, T.; Imai, H.; Furukawa, J. Makromol. Chem. 1962, 56, 55-64. (10) Lambert, J. L.; Goethals, E. J. Makromol. Chem. 1970, 133, 289-293. (11) Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1989; pp 229-230. (12) As for H:Si(111) substrate, we have detected insoluble film of the copolymer on the surface by FTIR and XPS (BCMO content was about 12%). While the film on graphite remained the same over a period of months, the film on H:Si(111) was unstable. We have observed a decrease in Cl2p intensity and a gradual increase of the SiO2 content as the samples have aged. Two weeks later, the film had completely peeled off. Although we did not perform STM to confirm epitaxial adsorption on silicon, we think that this instability is due to breaking of epitaxy by oxidation of the silicon surface. This assumption was also supported by the fact that if the silicon surface had been already oxidized before polymerization, no residual film on the silicon surface was detected even immediately after rinsing.
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Figure 2. An XPS spectrum obtained from the graphite sample. A peak at ca. 200 eV is due to Cl2p, which is unique to the copolymer. Other observable peaks are C1s (ca. 284 eV) and O1s (ca. 530 eV), respectively.
point in the range -150 to 100 °C. It is a viscous liquid at room temperature and is freely soluble in organic solvents used for rinsing. These results are consistent with a diblock form, PTHF-block-P(THF-BCMO), of the copolymer with some distribution in the block length. An FTIR spectrum from the graphite sample after rinsing is shown in Figure 1, together with a spectrum of a cast film of copolymer formed in solution on gold. The ethylene peaks around 2900 cm-1 and a strong ether peak at 1100 cm-1 indicate the polyether backbone. The film on graphite reproduces the main peaks of the block copolymer, with the exception of two additional peaks. A peak at 1589 cm-1 is due to an uncanceled part of the graphite substrate. A peak at 1080 cm-1 is assigned to a crystalline PTHF component9 and indicates that the PTHF block is crystallized on graphite but remains amorphous in a cast film. The differences in the relative intensity of individual peak are probably caused by the different orientation of molecules on each substrate and by a weak signal from the graphite surface that leaves an effect of water and a wavy baseline. The presence of BCMO component was confirmed by the Cl2p peak in XPS spectra as shown in Figure 2. Other observable peaks are C1s and O1s. The C1s peak was curvefitted to estimate the factions of graphite and polymer carbons. From the atomic ratios of C, O, and Cl, a fraction of BCMO in a whole polymer chain was calculated. Measurements on several separately prepared samples indicate that the polymer film on graphite contains 1317% BCMO component and the solution grown polymer has 12-20%, while the mole fraction of the starting BCMO monomer is about 20%. Thus, the insoluble film of the block copolymer chemically identical to the solution grown copolymer remained on the graphite surface even after washing. We obtained the similar results on H:Si(111), although the silicon surface was not as stable as that of graphite.12 The macroscopic feature of the film was investigated by AFM. Figure 3a displays a highly reproducible AFM images obtained from the graphite sample using the contact mode. A wavy netlike feature is a common characteristic covering more than 80% of the graphite surface. We interpret this wavy feature to be not a true morphology of the film but to contain a significant effect due to stick-slip motions of the cantilever forced to raster in a viscous film. Consistently, friction force image (not shown) also produced varying contrast at the positions of the netlike feature. This image, however, is consistent with the presence of an insoluble viscous film covering a significant portion of the graphite surface.
Figure 3. (a) Contact mode AFM images taken from a graphite sample showing a wavy netlike feature as well as both a bare graphite surface and a netlike feature. The graphite steps are also visible thorough the wavy feature as the straight lines running diagonally. The color scale is set so that a darker region appears higher. (b) Noncontact AFM image taken from the block copolymer sample on graphite. A straight line running diagonally at the lower left corner is a graphite step. The entire image is leveled so that all graphite terraces are approximately at the same height.
The same sample imaged by the noncontact AFM is displayed in Figure 3b. Patches of islands have a uniform “noncontact height” of 0.5 ((0.2) nm from the base plane. These island features are also seen in the PTHF homopolymer epitaxial film but not in the cast or meltcrystallized films of PTHF nor in the PBCMO homopolymer film.13 This observation supports an interpretation that these islands correspond to the crystalline PTHF block existing as an ordinary PTHF epitaxial layer. The number density of the visible islands, however, is considerably smaller in the block copolymer film than in the PTHF homopolymer film made under the same polymerization conditions. Because the effect of an ultrathin, viscous layer (tethered to the graphite surface) on AFM imaging is not known, it is not clear if there is less crystalline film on the surface or if only a part of the crystalline film is visible under the noncontact mode. (13) Sano, M.; Wada, M.; Miyamoto, A.; Yoshimura, S. Manuscript in preparation.
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and has felt smaller resistance by a reduction of dynamic viscosity at higher frequencies.14 The STM yielded three different types of images: the graphite lattice, periodic patterns that differed significantly from the graphite image (considered as molecular images), and unrecognizable, transient images. With the monomers used in the present study, the polymers that are able to form epitaxially adsorbed films are either PTHF homopolymer or PBCMO homopolymer. Pure P(THFBCMO) random copolymer was noncrystalline and was readily washed away from the graphite surface, giving no molecular image. We show an example of STM images observed on each polymer in Figure 4. Symmetry and the principal periods measured on the image patterns are as follows: rectangular, 12 and 4.3 Å, for the block copolymer (Figure 4a); rectangular, 12 and 4.3 Å for PTHF homopolymer6 (Figure 4b); oblique, 28 and 6.3 Å, 70° for PBCMO homopolymer15 (Figure 4c). This block copolymer image is previously identified as PTHF-I-0° of the epitaxial structures of PTHF in ref 6. Other structures such as PTHF-I-21.79° are also seen in the block copolymer film. Thus, the part that is observable by STM is the epitaxially adsorbed PTHF, which is consistent with the AFM observations. Since the polymer on the surface is diblock, the invisible P(THF-BCMO) block must be free of the crystalline attachment and may be responsible for the unrecognizable images. If a fresh graphite substrate was immersed in the reaction mixture after polymerization had been quenched and was rinsed similarly as above, no chlorine peak was detectable by XPS. Polymerizing only the noncrystalline block P(THF-BCMO) with the presence of graphite also failed to give any residual film on the surface as stated above. Thus, the insoluble PTHF-block-P(THF-BCMO) block copolymer could be made only by in situ polymerization of monomers with the crystalline PTHF block synthesized first. Conclusions
Figure 4. STM images (all in 70 × 70 Å2) taken from the graphite surface covered by (a) PTHF-block-P(THF-BCMO) copolymer, (b) PTHF homopolymer, and (c) PBCMO homopolymer. This image of the block copolymer is identified as an epitaxial structure of PTHF homopolymer, PTHF-I-0° in ref 6.
Molecular imaging of the crystalline part that is adsorbed directly on the surface was made possible by STM operating at a fast scanning rate (1-2 kHz) over a relatively small area (less than 10 × 10 nm2). Because the top layer is a viscous liquid at room temperature, the STM tip could penetrate through the top layer to reach within a tunneling distance from the surface. This caused the tip to be embedded in the viscous top layer, but its movement was apparently not affected at these scanning conditions. Thus, only a crystalline part that was fixed tightly onto the surface could be imaged. A possible explanation is that the tip has slid over the film without sticking at the short travel distance of this magnitude
The insoluble film of diblock copolymer was shown to form on the graphite surface during polymerization by first polymerizing the crystalline block, followed by polymerization of the noncrystalline block. This demonstrates that segmental epitaxy can be achieved by planning an order of polymerization according to the segment’s crystallinity through polymerization-induced epitaxy. Using this method, an amorphous segment could be tethered to the surface by epitaxial adsorption of a crystalline block. Noncrystallinity of the free block and insolubility of the whole film can be taken as the advantages to perform ordinary substitution reactions on the chlorine atoms of PBCMO to construct supramolecular structures. Acknowledgment. We thank M. Mikami and the members of Supermolecules ProjectsJRDC for their help. LA9505430
(14) Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, 1980. (15) Sano, M.; Sandberg, M. O.; Suzuki, M.; Yoshimura, S. Thin Solid Films, in press.
